Liquid crystal electric optical devices are used in a wide variety of products including portable equipment such as mobile telephones and PDAs; display units of office automation equipment such as photocopiers and monitors of personal computers; display units of home electric appliances such as liquid crystal televisions; and other products exemplified by watches, desktop calculators, measuring instruments, meters used in automobiles, and cameras, and are required to have various performances including a broad range of operating temperature, a low operating voltage, a high-speed response, and a chemical stability.
Such liquid crystal electric optical devices comprise constituent materials exhibiting a liquid crystal phase. Presently, however, not all of these properties are provided by a single compound and, therefore, a plurality of liquid crystal compounds and non-liquid crystalline compounds each excelling in one or more properties are mixed to meet the performances a liquid crystal compound is required to have.
It is a critical issue to provide a liquid crystal compound highly compatible with other liquid crystal materials or non-liquid crystal materials, chemically stable, and, when used in a liquid crystal electric optical device, capable of a high-speed response in a broad temperature range, and operable at a low voltage among various properties required of compounds used in a liquid crystal optical composition in the field of liquid crystal electric optical devices.
Particularly in recent years, a high response speed, i.e., improvement in response speed of liquid crystal electric optical devices are required in many applications while improving or maintaining other conditions at the same time: low-voltage operation and high speed response in battery-operated applications; high-definition display and high speed response in office automation equipment; and a broad range of operating temperature and high speed response at a low temperature in automobile meters. Such improvements are particularly important given the tendency of liquid crystals to exhibit a lower response speed as the temperature decreases.
Reducing the viscosity of a liquid crystal composition is one of the ways to improve the response speed. When the viscosity of a liquid crystal composition decreases, its response speed typically improves, and hence its response at a low temperature also improves. However, there have been no useful liquid crystal compositions thus far available with a sufficiently low viscosity. In addition to possessing such physical properties, a compound used in a liquid crystal composition is also required to excel in compatibility with another liquid crystal compound and/or non-liquid crystal compound, be a chemically stable compound, and achieve a low threshold voltage when used in a composition. To improve the response speed and achieve a low threshold voltage, selectively using liquid crystal compounds each having an appropriate elastic constant is of critical importance.
As a solution to such issue, a difluorostyrene derivative represented by formula I, for example, is considered (see Patent Document 1). However, the compound specifically described therein poses a problem that a fluorovinyl group is located adjacent a part of a conjugated system, which is liable to cause chemical instability.

On the other hand, a derivative represented by a following formula II (see Patent Document 2) or a compound having a structure Y-A-Cy-CF═CF2 (Cy:trans-1,4-cyclohexylene group, A:saturated or unsaturated ring group, Y:hydrogen atom, R or RO) (see Patent Document 3) are considered as a compound having a similar structure to that of the liquid crystal compound of the invention and of which the nematic phase exhibits a broad range of appearance temperature. However, specific liquid crystal compounds described in these documents had problems such that some of them have a univariable nematic phase and that their liquid crystal temperature range is small.
